Optical resonators are exemplary electro-optical devices that are often small in size, having diameters on the order of millimeters, and may be used in many optical system applications, including optical sensors for biological and chemical compounds, electro-optical oscillators and modulators, and tunable optical filters. The optical resonators are curved optical waveguides, i.e., a cylinder, a sphere, or a toroid within which light is internally reflected at the inner surface of the resonator. Optical resonators can support resonator modes of light called whispering-gallery modes (“WGMs”), and thus, are often referred to as whispering-gallery mode resonators. WGMs occur when light having an evanescent wave component travels via internal reflection around the periphery of the optical resonator. The evanescent waves extend beyond the optical resonator's outer surface and may be coupled into an adjacent optical coupler as long as the optical coupler is located within the extent of the evanescent wave, typically on the order of the light's wavelength.
Many optical resonators which propagate whispering-gallery modes of light have extremely low transmission loses, and as a result, have a very high quality factor (“Q”). High Q optical resonators are desirable because the higher the Q, the longer the amount of time the internally reflected light will remain within the optical resonator. The ultimate intrinsic Q of the optical resonator (Qo) is limited by the optical losses of the resonator material. Any practical coupling to whispering-gallery modes of the optical resonator can be accomplished through an evanescent wave from an adjacent optical element, i.e., an optical coupler.
If light from the optical coupler is over-coupled to the optical resonator, there will be broadening in the whispering-gallery mode output peak due to increased losses at the interface between the optical coupler and the optical resonator. If light from the optical coupler is under-coupled to the optical resonator, there will be less efficient energy transfer from the optical coupler to the optical resonator. Critical coupling occurs when enough energy is coupled from the optical coupler into the optical resonator to compensate for the roundtrip losses of the light propagating through the optical resonator. Coupling losses between the optical coupler and the optical resonator are exponentially dependent upon the distance d between the surface of the optical coupler and the optical resonator˜exp (−d/r*), where r* is the effective scale length of evanescent field of the resonator for the excited whispering-gallery mode as expressed in the following equation:r*=λ/√{square root over ((4π(nres/nout)2−1))}where                λ is the wavelength of light evanescently coupled between the optical coupler and the optical resonator;        nres is the index of refraction of the optical resonator; and        nout is the index of refraction outside the surface of the optical resonator.        
If the optical coupler contacts the optical resonator, too much of the light is evanescently coupled out from the optical resonator resulting in a low Q. Also, if the optical coupler is positioned far, more than three wavelengths, from the optical resonator, coupling of light between the optical resonator and the optical coupler becomes difficult. Thus, accurate positioning of the optical coupler relative to the optical resonator is critical to the efficiency of the optical system.
Optical couplers can be configured in various forms including those shown by example in FIGS. 1(a)-1(c) which include cross-sectional views, not shown to scale, of three different types of optical couplers 10, 12, and 14. In FIGS. 1(a)-1(c), each optical coupler is positioned adjacent to and spaced away from a cylindrical or spherical optical resonator 16, 18, and 20 by a distance “d”, which in practice is roughly on the order of the wavelength of the light to be evanescently coupled into or out from the optical resonator. Typically, d ranges in value from approximately 0.1 to 3 times the wavelength of the light. While not shown in FIGS. 1(a)-1(c), the optical resonator also may be toroidal in shape.
FIG. 1(a) shows an optical fiber coupler 10 that includes a core 22 and a cladding layer 24. The end of the optical fiber coupler closest to the optical resonator 16 has a flat polished surface 26 through which light is evanescently coupled into and out from the optical resonator. Similarly, FIG. 1(b) shows a prism coupler 12 having a flat surface 28 through which light is evanescently coupled into and out from the optical resonator 18. Also, FIG. 1(c) shows a tapered optical fiber coupler 14, again having a core 30 and a cladding layer 32, including a tapered section 34 through which light is evanescently coupled into and out from the optical resonator 20. In FIGS. 1(a), 1(b), and 1(c), incident light travels through the optical coupler as indicated by the straight arrows A1-A3, respectively, and internally reflected light travels around the periphery of the optical resonator as shown by the curved arrows B1-B3, respectively.
Because the optical resonator and optical coupler are small in size they may be integrated within small housings or devices that can be incorporated into various optical or electro-optical systems. However, one challenge associated with mass producing such integrated optical resonator and optical coupler combinations is providing for ease and repeatability in accurately setting and maintaining the exact separation for stable and exact strength of evanescent coupling. In the experimental setting, voltage-controlled piezo-positioners can be used to finely tune the positions of the optical coupler and optical resonator. However, the use of piezo-positioners is not conducive to mass production of optical systems employing optical resonators and optical coupler combinations. Thus, there is a need for a method of accurately separating an optical coupler relative to an optical resonator while maintaining a high Q.